Courtois made his senior international debut in October 2011, becoming the youngest goalkeeper to represent Belgium. He has since earned over 70 caps and appeared at the 2014 FIFA World Cup, UEFA Euro 2016 and the 2018 FIFA World Cup where he was awarded the Golden Glove as best goalkeeper of the tournament as the Belgians came third.

Courtois progressed through the Genk youth system, where Koen Casteels was initially regarded as the primary goalkeeper, but during an injury crisis Courtois, aged 16 years and 341 days, made his first team debut on 17 April 2009 against Gent. After turning down a transfer to TSG 1899 Hoffenheim of the Bundesliga, Courtois was made first-choice goalkeeper for the 2010–11 season ahead of Casteels, when Laszlo Koteles' registration ran into issues.[6]

For the 2013–14 season, Courtois' loan to Atlético was extended by a further 12 months.[26]

When Atlético were drawn to play Chelsea in the Champions League semi-final, it was reported that a clause in Courtois' contract required Atlético to pay Chelsea €3 million per match if they selected him against his parent club, and that Atlético could not afford such a sum. UEFA made it clear that considerations of sporting integrity made such a clause "null, void and unenforceable", and confirmed that Atlético were free to select Courtois without making any such payment.[27][28]

In June 2014, Chelsea manager José Mourinho confirmed that Courtois would return to Chelsea for the upcoming season.[31] He was assigned the number 13 kit number, last worn by Victor Moses.[32] On 18 August, Mourinho announced that Courtois would start the Premier League opener against Burnley in place of Petr Čech. Although he conceded the first goal to Scott Arfield at Turf Moor, Chelsea won 3–1.[33] Courtois kept his first Premier League clean sheet in his second game, making several crucial saves in a 2–0 home win over Leicester City.[34]

On 11 September 2014, Courtois signed a new five-year contract with Chelsea, keeping him at the club until 2019. Upon signing Courtois said: "It’s really nice to have signed this new contract for five years."[35]

Courtois suffered a head injury in the first half of Chelsea's 2–0 home win over Arsenal on 5 October due to a collision with Alexis Sánchez; he was substituted and then taken to hospital.[36] He was then treated for a minor cut to his ear and released from hospital that night.[37]

He won his first trophy with Chelsea on 1 March 2015, as they defeated Tottenham Hotspur 2–0 in the League Cup Final, with Čech instead playing in that match;[38] Chelsea also finished the season as league champions.

Courtois opened the season by playing in the 2015 FA Community Shield on 2 August, a 1–0 loss to Arsenal.[39] Six days later, as the Premier League campaign opened with a home fixture against Swansea City, he was given a straight red card for conceding a penalty with a foul on Bafétimbi Gomis, who converted the penalty past replacement Asmir Begović for a 2–2 draw.[40] On his return on 23 August, Courtois saved a penalty from James Morrison in a 3–2 win at West Bromwich Albion.[41] Courtois suffered a leg injury in training on 11 September which required surgery and meant that he was expected to miss the next three months of competition.[42]

On 16 April, he was sent off at home to Manchester City for conceding a penalty with a foul on Fernandinho; Sergio Agüero converted it past Begović to seal a 3–0 win. He became the sixth Premier League goalkeeper to be sent off twice in the same season.[43]

On 17 August 2016, Courtois dismissed any transfer rumours and stated he would stay at Chelsea for many years to come.[44] Ten days later in the 3–0 home victory over Burnley, Courtois kept the first clean sheet of the season and broke a run of thirteen home Premier League games without a clean sheet since November 2015.[45] From 1 October to 20 November, Chelsea earned a run of six games without conceding a single goal; with Courtois playing in all six of them.[46][47] From 11 to 26 December, Courtois kept four clean sheets, as Chelsea were at the top of table in time for Christmas.[48]

In April 2017, Courtois was ruled out for the match against rivals Manchester United, as a result of suffering a reported ankle injury, which he sustained in training.[49] Chelsea would go on to lose the match 0–2.[50] On 12 May 2017, Courtois kept his third consecutive clean sheet in a 1–0 away win over West Bromwich Albion, in which Chelsea secured the title.[51] Courtois also played for Chelsea in the FA Cup Final; which Chelsea lost 2–1 to Arsenal.[52]

Courtois kept a total of 16 clean sheets in the Premier League, and won the Golden Glove.[53]

Following the 2018 season, Courtois mentioned he would want to live closer to his children, reigniting the rumours about a transfer to Real Madrid.[54][55] After a strong performance at the 2018 FIFA World Cup, where he won the Golden Glove for best goalkeeper, Courtois expressed his interest to move on.[56][57] Chelsea stated they would not let him go, unless they would be able to find a replacement for the Belgian goalkeeper.[58] Courtois responded by not showing up on training after the summer break, trying to force his exit from Chelsea.[59] Finally on 8 August 2018, Real Madrid announced officially that they had signed Courtois for a six year contract.[60] A day later, Chelsea confirmed the transfer.[61]

He made his debut for Madrid on 1 September 2018, where he started in a 4–1 win over CD Leganés.[62]

Courtois was first called up to the Belgium squad in October 2011[63][64] and made his debut the following month in a friendly 0–0 draw against France at the Stade de France, making him the youngest goalkeeper to play for the Belgian national team.[65][66][67]

On 13 May 2014, Courtois was named in the squad to go to the 2014 FIFA World Cup.[69] At the tournament, he played all five games of the Belgian team, starting with a 2–1 win against Algeria in Belo Horizonte.[70] Courtois then managed to keep consecutive clean sheets in 1–0 wins against Russia and South Korea as the Red Devils reached the quarter-finals.

Courtois played every minute during Belgium's first eight games of their UEFA Euro 2016 qualification campaign, helping seal qualification for the first time in 16 years. However, he missed their last two games through injury.[71]

Courtois and his side made it to the quarter-final, where they lost to Wales despite taking an early lead. Afterwards, Courtois hinted that Belgium coach Marc Wilmots was at fault for the loss and also stated that the defeat was the "biggest disappointment" of his career.[72]

Courtois making a save during the quarter-final match v Brazil; which Belgium won 2–1, on 6 July 2018.

Courtois was selected to Belgium's final 23-man squad for the 2018 FIFA World Cup.[73] He was Belgium's first-choice goalkeeper throughout the tournament, and played a key role in Belgium's 2–1 win over Brazil in the quarter-final, which saw Belgium advance to the semi-finals of the tournament for the first time since 1986.[74][75] He posted two clean sheets in the group stage (against Panama and England), allowed only one goal in Belgium's loss to eventual champion France in the semi-final, and posted another clean sheet against England in the 3rd place game. Courtois made 27 saves in seven World Cup games, more than any other goalkeeper.[76] He was awarded the Golden Glove as best goalkeeper of the tournament.[77]

Considered a highly promising prospect in his youth,[78] Courtois subsequently established himself as one of the best players in the world in his position.[79] Due to his wide range of skills, he has been described as a "complete goalkeeper", with few weaknesses.[78] A consistent keeper,[78] he possesses an excellent positional sense,[80][81] good mentality, composure, strength of character,[80][82][83] and an ability to communicate well with his defenders;[78] he also excels in one on one situations, due to his ability to time his runs effectively when rushing out of goal to face opponents.[78][81] An excellent shot-stopper between the posts, he has also distinguished himself for his agility and quick reflexes, in spite of his size and imposing physique.[78][81][83][84][85] Due to his height and reach, he excels in the air,[78][86] and is also known for his command of his area, as well as his excellent technique, anticipation, handling, and confidence when coming off his line to claim crosses.[87][88]

On 26 May 2015, his Spanish girlfriend Marta Domínguez gave birth to their daughter, Adriana.[90] The couple ended their relationship in April 2017 while Domínguez was pregnant with their son Nicolás, who was born a month later.[91]

Inspired by Courtois' pose from a particular save in January 2013, a Colombian fan created a new widespread social media meme called 'Thibauting' to pay homage to the Belgian goalkeeper.[92] In November 2013, the word was included in a shortlist composed by leading Dutch dictionary Van Dale to be polled to determine the best new sports/amusement word of the year in Belgium,[93] and ended second.[94] The term is based on and pronounced the same way as "Tebowing", and is also similar to the worldwide 'planking' meme which was popular in 2011.

In February 2014, Courtois caused some stir in the national team, by saying of Simon Mignolet, his rival for the starting goalkeeper spot, "You have to know how to stay humble and respectful, and he should remember that."[95] This was despite the fact that in previous interviews, Mignolet had only said that it was his ambition to keep working and try to regain his spot in the national team.[96]

In April 2018, Wilmots, by then out of a job, accused Courtois and his father Thierry of leaking Belgium's line-ups before they should have been revealed to the public. He denied the allegations.[97][98]